The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The main elements of a spin valve can be seen in FIG. 1. They are lower shield/conductor layer 11 on which is (magnetically) free layer 12. Directly above the free layer is non-magnetic spacer layer 13 and above it is a (magnetically) pinned layer 14. Pinning of the latter is effected by antiferromagnetic (AFM) layer 15. Note that the pinned layer may be a single magnetically soft material such as NiFe or it could be a synthetic antiferromagnet formed by sandwiching an antiferromagnetic coupling layer between two antiparallel ferromagnetic layers. The topmost layer is magnetic shield layer 16 which also serves as a conductive lead for the device.
In the device illustrated in FIG. 1, the direction of current flow is shown as arrow 17. In other words, the current runs perpendicular to the plane of the device which is therefore referred to as a CPP device. It is also possible to arrange for the conductive leads to abut the vertical sidewalls of the GMR pedestal, in which case the structure becomes a CIP (current in plane) device. As track width grow narrower, the trend has been to favor CPP devices.
The device illustrated in FIG. 2 is essentially the same as the one seen in FIG. 1 except that the order of the elements making up the spin valve has been reversed. Both devices operate in the same way—When free layer 12 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will be at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a device of this type is typically 8-20%.
As magnetic recording densities get beyond 100 Gbpsi, the CPP GMR sensor becomes the reader of choice. It has the advantage of better signal-to-noise ratio and, also, its signal amplitude does not scale down with device dimensions which is a necessary quality as track densities get higher. However, the signal amplitude of a CPP head is generally too small to be practically useful, because of low dR/R. The present invention discloses how this shortcoming can be overcome.
This change of resistance, dR, can be detected as a signal voltage when a current passes through. Note that the sensing current in CPP mode is flowing perpendicular to GMR stack. dR, which represents a signal amplitude, depends on the materials chosen for the free and pinned layers as well as their neighboring layers. It also depends on geometry of the device. The material is normally characterized by β, the bulk spin asymmetry coefficient, and γ, the interface spin asymmetry coefficient. The dR contribution from β depends on how long electrons can interact in the bulk. Since the thickness of a CPP device is relatively small, being limited by the shield-to-shield spacing, it cannot contribute much and therefore the signal that can be detected is limited.
Current CPP GMR configuration suffers from an additional problem. The total device resistance comprises both the GMR and parasitic resistances, with the latter dominating. The result is degradation of dR/R to a very small value.
A routine search of the prior art was performed with the following references of interest being found:
GMR structures are disclosed in U.S. Pat. No. 5,576,914 (Rottmayer et al) and in U.S. Pat. No. 6,084,752 (Sakakima et al). In U.S. Pat. No. 5,668,688, Dykes et al describe a conductor on top of the pinning layer while Yuan et al. show conductor layers adjacent to each end of the GMR in U.S. Pat. Nos. 6,219,205 and 5,739,987.